This invention concerns a system useful with instruments that are capable of measuring and indicating the total electric current that has been conducted through an electrical circuit. It is particularly useful with electrochemical devices known as coulometers and therefore will be described in conjunction with such devices.
Coulometers are described in detail in Lester Corrsin's U.S. Pat. No. Re. 27,556 entitled "Operating Time Indicator" and Curtis Beusman's U.S. Pat. No. 3,193,763 entitled "Electrolytic Coulometric Current Integrating Device," both of which are incorporated herein by reference.
The device described in these patents includes a tubular body of nonconductive material having a bore therethrough that supports two columns of a liquid metal such as mercury. The adjacent innermost ends of these columns are separated by a small volume of electrolyte with which they make conductive contact. The outermost ends of the liquid metal columns contact conductive leads that connect the instrument to the source of electric current that is to be measured. In accordance with Faraday's Law, when current flows through the instrument, liquid metal is electroplated from the anode column to the cathode column, causing the anode to decrease in length and the cathode to increase an equal amount, the change in column length being directly proportional to the total electric charge passed through the instrument.
Readout of the total current through the instrument may be made by comparing the length of a column against a calibrated scale. Typical visual readout devices are described in the above-identified Corrsin patent and in Beusman's U.S. Pat. No. 3,343,083 entitled "Nonself-Destructive Reversible Electrochemical Coulometer." It has also been found that the coulometer may be read out electrically by measuring changes in the capacitance between the mercury columns and an electrode surrounding the tubular body. The details of such a readout device are set forth in Edward Marwell and Curtis Beusman's U.S. Pat. No. 3,255,413 entitled "Electro-Chemical Coulometer Including Differential Capacitor Measuring Elements" and Eugene Finger's U.S. Pat. Nos. 3,704,431 and 3,704,432 entitled "Coulometer Controlled Variable Frequency Generator" and "Capacitive Coulometer Improvements," respectively, all of which are incorporated herein by reference.